Heretofore, in the case of keeping the humidity within a controlled chamber constant by using an ultrasonic humidifier, the humidity within the controlled chamber has been controlled by such ON-OFF-operations as are shown in FIG. 6.
That is to say, as shown in FIG. 6, an ON-humidity H.sub.ON and OFF-humidity H.sub.OFF having a hysteresis .DELTA.H for a objective humidity H.sub.O are determined, the humidity within the controlled chamber is detected with a humidity detector, at the time points t.sub.1 and t.sub.3 when the detected humidity H reaches the OFF-humidity H.sub.OFF while rising, the oscillation of the oscillator of the ultrasonic humidifier is perfectly stopped and, at the time points t.sub.2 and t.sub.4 when the above mentioned detected humidity H reaches the ON-humidity while lowering, the oscillator of the ultrasonic humidifier is oscillated with an amplitude of 100%.
By the way, in the case shown in FIG. 6, the detected humidity H at the control starting time point t.sub.o is so low that the oscillator of the ultrasonic humidifier oscillates with an amplitude of 100%.
However, as the place of setting the ultrasonic humidifier and the place of setting the humidity detector are usually separated from each other, the time of transmitting air humidified by spray is comparatively long and the time of being sensed by the humidity detector itself can not be neglected. Therefore, that is to say, in the humidification controlling system, the transmission delay and wasted time are generally so large that, with the above mentioned conventional humidification controlling system, there have been defects that, as shown in FIG. 6, an overshoot .DELTA.A and undershoot .DELTA.B will be generated, therefore a cycling (continued oscillation) of a large amplitude will occur and the humidity within the controlled chamber will not be able to be precisely kept constant.
Also, in the above mentioned conventional humidification controlling system, as described above, the hysteresis .DELTA.H is provided to somewhat reduce the frequency of the ON-OFF-operations (the oscillation of 100% amplitude and the perfect stop of the oscillation) of the oscillator of the ultrasonic humidifier. However, the precision will be further reduced by the part of the hysteresis .DELTA.H, therefore the hysteresis will not be able to be made so large and therefore the above mentioned oscillator will be so high in the ON-OFF frequency as to be likely to be broken.